Exactly what i did am doing last weekend today! Mine was attached to a
toilet, that is being replaced, may not be relv or suitable in your
postition, but i found cutting as much aroun the pipe as possible using an
angle grinder then with fairly big hammer whacked the section in the middle
and it simply broke, thereby allowing me to cut the bit at the back, when i
say simply i dont mean simply it does take a few (alot) of hits with the
hammer (wear eye protection) If your just replacing a section and its
visible consider using cast iron, appar, its not expensive and easy to use,
also you need to make sure there is sufficient support for the pipe above if
your putting a section in as plastic wont support the weight of cast iron,
(a tiny section i cut out weighed a tonn)
Have fun
Olly